1. Field of the Invention
The present invention relates to a thin film transistor-liquid crystal display (TFT-LCD), and more particularly, to a pixel array structure of a TFT-LCD.
2. Discussion of Related Art
In a TFT-LCD of an active matrix liquid crystal display (AMLCD) driven by a dynamic driving method, an active device, for example, TFT, is formed at each of pixels in the LCD. The active device is used to control each pixel individually. A gate driving circuit and a data driving circuit are a formed in the peripherical region of a TFT matrix array, that is, a pixel array in which data output display is formed. The driving circuits are built-in or attached to a separate circuit board. Here, the driving circuits are formed of TFTs having complementary structures to tolerate a higher voltage.
The gate driving circuit includes a shift register having a plurality of output lines, and a buffer having a plurality of input/output lines which are connected to the output lines of the shift register. Each output line of the buffer is connected to a scanning line of the pixel array.
The data driving circuit includes a shift register having a plurality of output lines, a buffer having a plurality of input/output lines which are connected to the output lines of the shift register, and a plurality of switching devices which are connected to the output lines of the buffer and driven by a signal input from the buffer output lines. Each switching device connects each signal line of the pixel array to a data signal input line.
The data signal applied to the signal line is turned on by a gate driving pulse generated from the gate driving circuit and transmitted to the liquid crystal and storage capacitor of the pixel through a TFT formed at the intersection of a scanning line and an activated signal line. Here, the storage capacitor receives the data signal together with the liquid crystal when the TFT is turned on, and helps the liquid crystal to maintain the signal.
In order to prevent the liquid crystal from deteriorating, the LCD is AC-driven. That is, positive and negative signals are alternately applied to the liquid crystal of each pixel according to a predetermined cycle. However, since the voltage variations in the positive and negative signals are different from each other, the AC-driving method makes the effective voltage applied to the liquid crystal unstable. Thus, the amount of light through each pixel varies, resulting in a flicker phenomenon of the final picture.
To prevent such flicker phenomenon, a data inversion method has been proposed. In this method, a data signal is applied to a predetermined pixel as positive and negative signals alternately. The data inversion may be divided into frame inversion, line inversion, column inversion and dot inversion. With the frame inversion, the positive and negative signals of the data signal are alternately applied to the whole pixel array according to each frame (field) so that the light transmissivity over all the pixels is changed whenever a picture is changed.
With the line inversion, the signals are alternately applied according to each of the scanning lines, so that the light transmissivity on each scanning line of the matrix array changes alternately. With the column inversion, the signals are alternately applied according to each of the signal lines, so that the light transmissivity is alternately changed on each signal line.
With the dot inversion, which is a combination of the line inversion and column inversion, the signals are alternately applied to make the polarities of neighboring pixels opposite to each other. By doing so, the neighboring pixels take on light transmissivities different from each other. The data signal input method using the dot inversion can minimize the flicker because of the spatial averaging of the whole picture.
FIG. 1 shows a general LCD panel. A plurality of signal lines D1, D2, D3, etc. and scanning lines G1, G2, G3, etc. cross each other to form a matrix pixel array. A signal line and a scanning line are connected to each pixel, and a pixel electrode is connected to the drain electrode of TFT of each pixel. That is, the pixel electrode forms the matrix array. Here, either n-type or p-type TFTs are formed in all the pixels. In FIG. 1, n-type TFTs are employed, and a common voltage of each pixel is ground voltage. In the peripheral portion of the pixel array, a gate driving circuit 10 is connected to the scanning lines G1, G2, G3, etc. to drive the TFTs, and a data driving circuit 20 is connected to the signal lines D1, D2, D3, etc. to apply a data signal to the TFTs.
A conventional dot inversion method will be explained below with reference to FIGS. 2 and 3. FIG. 2 shows a data driving range of the data driving circuit. In this case, the common voltage (Vcom) of each pixel is fixed at a DC voltage. In addition, the common voltage may be selected by a circuit designer. FIG. 3 shows the polarity of a data signal charged in the pixels in a predetermined frame (field) according to the dot inversion method for 4.times.4 matrix pixels of the LCD panel. Here, "+" represents a region ranging from V3 to V4, "-" represents a region from V1 to V2 in FIG. 2. When a predetermined scanning line is activated and the next scanning line is thereafter activated, that is, whenever the column of the activated scanning line changes, the polarity of the data signal changes in the data driving circuit.
Accordingly, on one signal line, the voltage polarity changes from the range V1 through V2 to the range V3 through V4 ,or from the range V3 through V4 to the range V1 through V2 whenever the row is changed. This, however, widens the voltage variation, resulting in an increase of power consumption.